Add GitHub CI workflow for Modal tests

Runs cargo tests on Modal using offload-cargo-modal.toml config.
Requires MODAL_TOKEN_ID and MODAL_TOKEN_SECRET secrets.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jacobkirmayer-imbue

.github/workflows/test-modal.yml

@@ -0,0 +1,39 @@
+name: Tests (Modal)
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Install Rust
+        uses: dtolnay/rust-action@stable
+
+      - name: Cache cargo
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.cargo/bin/
+            ~/.cargo/registry/index/
+            ~/.cargo/registry/cache/
+            ~/.cargo/git/db/
+            target/
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+
+      - name: Install uv
+        uses: astral-sh/setup-uv@v4
+
+      - name: Build offload
+        run: cargo build --release
+
+      - name: Run tests on Modal
+        env:
+          MODAL_TOKEN_ID: ${{ secrets.MODAL_TOKEN_ID }}
+          MODAL_TOKEN_SECRET: ${{ secrets.MODAL_TOKEN_SECRET }}
+        run: ./target/release/offload -c offload-cargo-modal.toml run